Some modern radar systems use planar arrays of antenna elements to transduce electromagnetic energy between guided and unguided forms. It is difficult to obtain hemispheric coverage with a single planar array, so it has become common practice to use a plurality of generally co-located, co-acting planar arrays, each providing coverage of a portion, such as a quarter, of a hemisphere. The transmission and reception of signals from the various planar arrays are coordinated in order to avoid, inasmuch as possible, large steps in apparent track position when a target moves from the coverage region of one planar array to that of another.
The various co-acting planar antenna arrays of a radar system intended for covering a broad region cannot be at precisely a prescribed orientation. Thus, the planar arrays must be physically supported in close proximity to each other, and with precisely accurate as-installed measured relative angular orientations. This is accomplished by an “internal” structure. Put another way, there must be an accurate relative alignment between multiple planar radar arrays internal to the structure supporting such arrays for the purpose of engaging and/or observing a plurality of targets in a hemispherical environment. A singular measurement may not be sufficient for some applications because the structure will expand and contract or flex abnormally under environmental conditions. This flexure and expansion/contraction of the various portions of the antenna array support structure can be expected to adversely affect the operation of the radar system as a whole.
In some cases, antenna arrays of a radar system may be mounted on the superstructure of a ship. In such systems, legacy measurements of the alignment of the antenna arrays have been made by means of one reference theodolite mounted at the ship centerline on an exterior weather deck and another theodolite mounted in the proximity of an antenna array from which it may note the locations of particular exterior antenna-related scale targets by sweeping a plane with a right angle attachment and generating reference angles from the reference theodolites. Each theodolite is therefore referenced to a ship specific centerline and horizontal reference for these measurements.
More recent methods of alignment substitute a laser tracker for the theodolite in proximity to an antenna array, and an optical retroreflector for the scale targets. The laser tracker still has to pick up the same centerline and horizontal plane references. The laser tracker, unlike the theodolite, can precisely locate the retroreflector target positions in angle and distance. Naturally, the location of the target or retroreflector must be selected to be visible from the location of the tracking instrument. However, because the antenna arrays face in different directions, some of the antenna arrays will be hidden from the view of an instrument placed in proximity to the exterior of the antenna array, and in that case additional instruments may be required at other locations, in order to complete the measurements of all of the arrays.
Improved or alternative alignment methods are desired.